1. Field of the Invention
The invention concerns the field of reaction injection molded polyurethanes (RIM).
2. Description of the Prior Art
Reaction Injection Molding (RIM) is a technique for the rapid mixing and molding of large, fast curing urethane parts. RIM polyurethane parts are used in a variety of exterior body applications on automobiles where their light weight contributes to energy conservation. RIM parts are generally made by rapidly mixing active hydrogen containing materials with polyisocyanate and placing the mixture into a mold where reaction proceeds. These active hydrogen containing materials comprise a high molecular weight polyhydric polyether and a low molecular weight active hydrogen containing compound. In the invention to be described below, the low molecular weight active hydrogen containing compounds are either ethylene glycol or 1,4-butane diol. Generally, the active hydrogen containing materials, both high and low molecular weight, are mixed together with catalyst and other optional materials in one tank and the polyisocyanate is contained in another tank. When these two streams are brought together in a mold, the RIM part is made. One problem with prior art processes is that the high molecular weight polyhydric polyether and the ethylene glycol or 1,4-butane diol are incompatible and will not form a stable emulsion. This leads to processing difficulties since a stable emulsion is necessary for a consistently uniform RIM part to be made.
U.S. Pat. No. 3,489,698 describes a prior art system wherein a high molecular weight polyol having a hydroxyl number range of 56 to 34 and a low molecular weight polyol having a hydroxyl number range of from 420 to about 650 was used. These materials of differing molecular weights were found to be incompatible and block copolymers of ethylene and propylene oxide ranging in molecular weights from 5,000 to about 27,000 and having functionalities of 2 or 3 were used to stabilize these emulsions of 2 polyols. In a paper given in 1974 by the same assignee of the above U.S. patent (Union Carbide Corporation), "Plastics in Surface Transportation", National Technical Conference, Society of Plastics Engineers, 1974, pp. 64-68, a complex system is offered wherein a 6,000 molecular weight polyol was made into a stable emulsion with ethylene glycol by the use of a di-ethylene oxide adduct of aniline and approximately equal weight ratios with the ethylene glycol. The resulting mixture of ethylene glycol and the di-ethylene oxide adduct of aniline was compatible with the high molecular weight polyol.
It has been surprisingly discovered that a much simpler and direct route to compatibility of ethylene glycol and 1,4-butane diol with high molecular weight polyols may be achieved by the use of particular high molecular weight ethylene oxide/propylene oxide copolymer diols having a high content of ethylene oxide.